Screen plate for centrifugal pulp screens



Nov. 5, 1968 E. B. MEA DOWS SCREEN PLATE FOR CENTRIFUGA L PULP SCREENS Filed Jan. 5, 1966 2 3% M w M M ac? M W EW w.

United. States Patent 3,409,132 SCREEN PLATE FOR CENTRIFUGAL PULP SCREENS Ewell B. Meadows, Memphis, Tenn., assignor to The Buckeye Cellulose Corporation, Cincinnati, Ohio, a corporation of Ohio Filed Jan. 3, 1966, Ser. No. 518,197

3 Claims. (Cl. 209273) ABSTRACT OF THE DISCLOSURE An improved, non-blinding screen plate for use in apparatus for the centrifugal screening of fibrous slurries. The improved, non-binding screen plate is provided with raised longitudinal bars having fiber passing orifices disposed between leading and trailing edges on the raised interior surfaces thereof. The raised longitudinal bars derive their non-blinding characteristics from the hydrofoil surfaces presented to rotary fiber slurry motion within the centrifugal screening apparatus.

This invention relates to an improvement in apparatus for the centrifugal screening of fibrous slurries. More particularly, the invention concerns a non-blinding screen plate with improved fiber passing capacity for use in fibrous slurry screening apparatus equipped with a rotating member to induce a rotating centrifugal motion in fibrous slurries. Specifically, the invention consists of a screen plateprovided with raised longitudinal bars, provided with fiber passing orifices, disposed on the interior surface thereof for use in the screening of cotton linter fiber slurries, Wood pulp fiber slurries and the like.

It has heretofore been proposed to prevent the plugging or blinding of the fiber passing orifices in the screen plates of fibrous slurry screens by subjecting either the screen plates, or a movable metal or rubber diaphragm placed adjacent thereto, to a vibratory motion. It has also been proposed to shape the blades of the rotors in a cylindrical screen plate for fibrous slurries to result in the inducement of orifice cleaning turbulences. For example, US. 2,845,- 848, issued to Robert E. Bowen on Aug. 5, 1958, discloses a method of sloping rotor blades which alternately lifts the fibers toward the screen plate and then pulls them away by rotor blade turbulence. US. 3,029,951, issued to Madison M. Cannon on Apr. 17, 1962, discloses a cylindrical screening device in which the rotator is provided with foil-vane assemblies in which the foil provides a turbulent screen plate cleaning action and the vane directs the material loosened from the screen plate to a rejects outlet.

The rotor blade induced screen plate cleaning action, however, has not proven completely satisfactory in screening fibrous slurries containing longer fibers or fibers which possess a comparatively greater screen-blinding tendency. For example, fibrous slurries containing undigested or raw second-cut cotton linters have exhibited notably poor screening characteristics in cylindrical pulp screens, both as to screen plate orifice blinding and as to screen capacity. Fibrous slurries containing pine wood fibers have also proven difficult to screensatisfactorily with conventional screen plates. Indeed, fibrous slurries containing fibers of equal or greater length than those found in second-cut cotton linters have been impractical to screen using the aforementioned equipment.

Another disadvantageous point in the operation of centrifugal pulp screens has existed in the area of a lower than desired screen capacity as measured either in terms of weight units of fiber through-put versus screen plate area or as measured in terms of. the power required to ice turn the rotor for a given weight unit of fiber throughput.

Accordingly, it is an object of this invention to provide means for overcoming the aforementioned problems and disadvantages.

Another object is to provide a screen plate which is effective in screening fibrous slurries containing relatively long fibers for use in centrifugal pulp screens.

A further object is to provide -a screen plate for use in centrifugal fiber screening apparatus which screen plate substantially eliminates screen plate orifice blinding caused by the stapling and 'bradding of fibers in the orifices of the screen plate.

A still further object is to provide a screen plate for use in centrifugal pulp screens which induces a continuous self-cleaning action upon passage of rotor blades.

The objects of the present invention are achieved with a screen plate equipped with raised longitudinal bars having fiber passing orifices disposed on the interior raised surfaces thereof for use in apparatus for the centrifugal screening of fibrous slurries, which screen plate has improved self-cleaning tendencies.

The above and other objects and advantages of this invention will be made clear from the following detailed description and by the accompanying drawings in which:

FIGURE 1 is a vertical section through the axis of a typical centrifugal screening apparatus, employing either a conventional screen plate or a screen plate of this invention.

FIGURE 2 is a vertical section taken along line 2-2 of FIGURE 1.

FIGURE 3 is a partial enlarged cross-section of a conventional screen plate with a smooth interior surface, taken along the line 3-3 of FIGURE 1.

FIGURE 3a is a partial enlarged cross-section of one embodiment of the screen plate of this invention, taken along line 3-3 of FIGURE 1.

FIGURE 4 is an enlarged partial isometric view of the top surface of several longitudinal screen plate bars as shown in FIGURE 3a which illustrates the position of the fiber passing orifices on the raised longitudinal bar surface.

FIGURE 5 is an end View of a single longitudinal bar of this invention illustrating the angular relationships of the surfaces thereof.

In describing the preferred embodiment of the invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. It is not intended, however, to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.

In the liberation and bleaching of fibers for use in papers, textiles, rope and dissolving pulps, a screening operation serves the purpose of removiiig' coarse fibers, bark, hulls, pieces of stem, incompletely digested pieces of Wood and other material foreign to the desired fiber product. A screening operation can also be used either to classify fibers according to length or to separate comparatively long fiber for recycle to a refining or fiber-cutting step. In general, a screening operation is employed to remove particles which would carry through a sequence of bleaching stages to appear as specks, dirts, chemically inactive particles or weak points in ultimate papers, nonwoven fabrics, regenerated fibers and films and the like prepared from the fibers of the fibrous slurry.

The present invention is illustrated herein by reference to the screening of an aqueous slurry second-cut cotton linters having a fiber consistency of about 0.5% to about 3% or containing about 0.5 to about 3% by weight of cbtton linters 'in their raw state prior to digestion. Raw, second-cut cotton linters have an average fiber length of about 2 mm. to about 3 mm. and have proven extremely difficult to screen effectively because of their screen plugging or blinding characteristics. The disclosed screen plate construction has also proven effective in increasing the capacity of a centrifugal pulp screen operating on an aqueous slurry containing about 0.5% to about 3% by weight of sulfate digested pine fibers having an average fiber length of about 2 mm.

Although the invention is illustrated by reference to' specific fiber screening operations, persons skilled in the art will understand that fiber concentration, length, flexibility, diameter, etc. will vary in fibrous slurries to be screened. Therefore, while screen plate areas, rotor speeds, size patterns and shapes of screen perforations, etc. will necessarily vary with the type of fiber slurry being screened and the results desired according to capacity or fiber classification, the disclosed screen plate construction, having the screen plate orifices disposed through the raised surfaces of longitudinal bars placed axially in relation to the rotor blades of a centrifugal pulp screen, will enhance screen operation.

The aqueous slurries, respectively of cotton linter and sulfate pine fibers, used illustratively herein are representative of difficult screening problems with regard to capacity, screen-blinding and separation, so that the illustrative dimensions and consistencies, etc. will enable those skilled in the art to arrive at optimum conditions for particular fibrous slurry screening problems using the novel screen plate of this invention. Examination of the disclosed screen plate construction after screening aqueous slurries of second-cut cotton linters has revealed substantially no plugging or blinding of the screen plate perforations due to bradding, stapling or obstruction by other causes after extended continuous operation. Conventional flat surface screen plates having substantially the same orifice size and spacing were found to plug or blind almost immediately in the same screening service. The non-blinding characteristics of the disclosed screen plate have also resulted in increased capacity in screening aqueous slurries of sulfate pine fibers.

Referring now to FIGURES 1 and 2, there is shown a generalized drawing of a centrifugal pulp screen wherein the screen plate of the present invention can be mounted. The centrifugal pulp screen includes an outer casing having end plates 13 equipped with flanges 14 to which is attached a screen plate 11 in the form of a perforate cylinder. Other conventional means of screen plate mounting are satisfactory for use with present screen plate, and the cylindrical screen can be made up of individual screen plate sections. The screen plate encloses a rotor assembly 12. FIGURES 1 and 2 illustrate the essential components of a centrifugal pulp screen and the position of the screen plate therein for the purpose of describing the contemplated screening operation. It is understood, however, that the screen plate of this invention is useful in any centrifugal pulp screen, having a rotor with blades spaced in close proximity to the screen plate, including, for example, the centrifugal pulp screens disclosed and claimed in U.S. 2,246,669, issued to Elijah Cowan on June 24, 1941 and in U.S. 3,029,951 issued to Madison M. Cannon on Apr. 17, 1962.

In particular, the non-blinding and plugging avoidance results disclosed herein were obtained by mounting the screen plates provided with the disclosed longitudinal raised bars in an Improved Machinery, Inc., Model A-25 Centrifugal Pulp Screen as described in that companys bulletin B2-1, No. 0559 and containing a rotor similar to that disclosed in U.S. 2,845,848, issued to Robert E. Bowen on Aug. 5, 1958.

FIGURE 3 shows a partial enlarged cross-section of a conventional screen plate and thereby illustrates the perforate nature of a conventional screen plate and the smooth surface provided on the interior surface of the screen plate, adjacent the rotor blades of a centrifugal pulp screen. FIGURE 31:, in contrast, shows an enlarged cross-section of the novel screen plate disclosed herein, illustrating the cross-section of the inwardly projecting, longitudinally extending perforate surfaces or bars having orifices therein provided on the surface of the screen plate adjacent the rotor.

FIGURES 4 and 5 show further enlarged views of the screen plate depicted in FIGURE 3a. Thereis shown a section of screen plate comprising a multiplicity of inwardly projecting raised surfaces or longitudinal bars having fiber passing orifices therethrough which longitudinal bars arespaced from one another by imperforate channels. Each of the imperforate channels in the screen plate is defined in width by a pair of generally outwardly directed sidewalls, 18 and 20, one of which extends from the leading edge of the raised surface 19 of a longitudinal bar and the other of which extends from the trailing edge of said surface. In the embodiment of the invention depicted in FIGURE 4, and particularly in FIG- URE 5, the sidewall 18 which extends from the leading edge of the raised longitudinal bar surface forms an included angle a of about with the raised surface 19 from which it extends, although the included angle a can be effectively varied from about 60 to about Similarly, the sidewall 20 which extends from the trailing edge of the raised longitudinal bar surface 19 forms an included angle ,8 of about with the raised surface 19 from which it extends, although the included angle ,8 can be effectively varied from about 110 to about FIGURES 4 and 5 also illustrate that the screen plate is provided with radially extending orifices or perforations in a multiplicity of locations lying within the bounds of the raised longitudinal bar surfaces. Applicant has satisfactorily used orifice diameters of about inch to about inch in screening cotton linter slurries and also pine fiber slurries. In screening pine fiber slurries the effectiveness of the present screen plate construction was surprisingly demonstrated by the fact that the throughput of the test screen was substantially doubled by the replacement of conventional fiat screen plates by plates of the present construction even though the replacement plates had substantially fewer fiber passing orifices of smaller (0.0625 inch diameter) than did the conventional screen plate (0.075 inch diameter). Also, and even more surprisingly, repeated tests revealed virtually no bradding, stapling or other forms of fiber blinding when screening aqueous slurries of cotton linters, even though conventional screen plates with comparably sized fiber passing orifices blinded almost immediately in the same screening service. Bradding will be understood by those skilled in the art to be the condition wherein a fiber is lodged against a screen plate with one of its ends extending downwardly into a fiber passing orifice; stapling connotes a similar fiber lodgement in which both ends of the fiber extend into fiber passing orifices, while blinding refers to any buildup of material which blocks screen openings. In addition to the foregoing results replacement of conventional flat screen plates by screen plates of the present construction resulted in appreciably cleaner fiber accepts in each instance as measured by Technical Association of The Pulp and Paper Industry dirt count techniques in comparison with both the dirt counts of the unscreened fibrous slun'ies and the dirt counts of fibrous accepts obtained through the replaced conventional screen plates.

Applicant has further found that transverse width of the raised surface of the longitudinal bars can range from about inch to about /2 inch, although a raised bar surface of about inch wide is preferred for the screening applications investigated. The heighth of the longitudinal bars from their raised surfaces to the imperforate channels therebetween has ranged from about 3 inch to about inch, with about /8 inch being preferred in the investigated screening operations. Also, the width of the imperforate channels between the raised longitudinal bars has ranged from about /1 inch to about 1 inch with 1 inch being preferred in the investigated screening operations. It will be understood by those skilled in the art that the above longitudinal bar and channel dimensions are subject to wide variation depending on the fiber lengths, slurry consistencies and other factors in the screening operation in which the screen plates are to be employed.

Although applicant has used fiber orifice diameters of about 4 inch to about 7 inch, as mentioned hereinbefore, and prefers to use fiber passing orifice spacings which place said orifices as far apart as is consistent with provision for as much orifice area as possible, and which spacings are as nearly the average fiber length being screened as possible, all conventional fiber passing orifice sizes and spacings can be used in screening practice with the presently disclosed screen plate construction. For example, although the invention has been illustrated with fiber passing orifices in the form of round, drilled or punched holes, which can be relieved on the surface of the screen plate opposite the rotor blades as in modern practice for conventional screen plates, the fiber passing orifices can take the form of slots in the outward surfaces of the longitudinal bars and extending the full length thereof, or, alternatively, there may be provided a pattern of intermittent slots or elongated holes having their smallest dimension in keeping with good screen practice for the type of material being screened. Also, it will be understood by those skilled in the art that the longitudinal bars disclosed herein for use on the interior surfaces of screen plates can be independently fabricated and then mounted on the surface of flat or curved plates prior to drilling or otherwise machining the fiber passing orifices through both the longitudinal bars and the plates upon which they are mounted. The mounting of the longitudinal bars can be made by any suitable technique available in the art, for example, the bars can be welded to the mounting plate or they can be drilled and the mounting plate drilled and tapped so that the mounting can be accomplished by the use of fiat-headed machine bolts. It will also be understood that the screen plates including the raised longitudinal bars can be self-machined from one piece of metal, although this method of fabricating the disclosed screen plates will be found to be comparatively more expensive than individually mounting the longitudinal bars.

In operation the fibrous slurry enters the centrifugal pulp screen through inlet into the chamber enclosed by the cylindrical screen plate 11. The rotor, rotating in the direction shown in FIGURE 2, whose plurality of radially oriented impeller blades have distal edges adapted to sweep past the innermost surfaces of screen 11 in closely spaced relationship therewith, picks up the entering slurry and provides it with a rotary and centrifugal motion.

Accepted fibers, passing through the screen 11, emerge into the chamber formed by the outer casing 10 and the screen 11 and through the accepts outlet 16. Particles and fibers, etc. rejected by screen 11 travel along the length of screen 11 and emerge at the rejects outlet 17.

While particular embodiments of the invention have been illustrated and described, it will be obvious to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention and it is intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.

What is claimed as new is:

1. A screen plate for use in the centrifugal screening of fibrous slurries, said screen plate having on the interior surface thereof a multiplicity of projecting, longitudinally raised surfaces spaced from one another by imperforate channel portions, each of said channel portions being defined in width by a pair of generally outwardly directed sidewalls, one of which extends from the leading edge of each of said raised surfaces and the other of which extends from the trailing edge of each of said raised surfaces and said raised surfaces being per-forated in a multiplicity of locations lying within the bounds of each of said raised surfaces wherein the sidewall which extends from the leading edge of each of said raised surfaces and the raised surface from which it extends includes an angle of about 60 to about 140, and the sidewall which extends from the trailing edge of each of said raised surfaces and the raised surface from which it extends includes an angle of about to about 170, wherein the angle included at the leading edge of said longitudinally raised surfaces is smaller than the angle included at the trailing edge.

2. A screen plate for use in the centrifugal screening of fibrous slurries as defined in claim 1 wherein the sidewall which extends from the leading edge of each of said raised surfaces and the raised surface from which it extends includes an angle of about and the sidewall which extends from the trailing edge of each of said raised surfaces and the raised surface from which it extends includes an angle of about 3. A centrifugal fiber screening device comprising in combination an outer casing providing means for the introduction and discharge of fibrous slurries, a stationary cylinder formed by screen plates enclosed within said casing and an impeller rotatably mounted within and coaxial of said cylinder and casing and supporting a plurality of radially oriented impeller blades, the distal edges of which blades are adapted to sweep past the surfaces of the interior of said cylinder in closely spaced relationship therewith, the interior of said cylinder comprising a multiplicity of inwardly projecting, longitudinally extending raised surfaces spaced from one another by imperforate channel portions, each of said channel portions being defined in width by a pair of generally outwardly directed sidewalls, one of which extends from the leading edge of each of said raised surfaces and the other of which extends from the trailing edge of each of said raised surfaces and said cylinder being radially perforated in a multiplicity of locations lying within the bounds of each of said raised surfaces, wherein the sidewall which extends from the leading edge of each of said raised surfaces and the raised surface from which it extends includes an angle of about 135, and the sidewall which extends from the trailing edge of each of said raised surfaces and the raised surface from which it extends includes an angle of about 150.

References Cited UNITED STATES PATENTS 2,185,955 l/ 1940 Schibbye 209-397 2,311,813 2/1943 Beck 209-506 X 2,465,056 3/ 1949 Bond 209-397 X 2,908,390 10/ 1959 Rich 209-273 FOREIGN PATENTS 2,385 6/ 1876 Great Britain. 273,215 12/1912 Germany.

HARRY B. THORNTON, Primary Examiner.

R. HALPER, Assistant Examiner. 

